This invention relates to a television receiver.
In a prior-art television receiver, in order to form video signals of fixed level which are not affected by the changes of an input field intensity, the gains of a video intermediate frequency amplifier stage and a radio frequency amplifier stage are controlled with an AGC voltage proportional to the peak value of a synchronizing signal. Since an AGC circuit for forming the AGC voltage responds also to noise exceeding a synchronizing signal level, a noise canceller is required.
In case of constructing the noise canceller so that signals exceeding a fixed detection level may be regarded as noise and may have their transmission inhibited or be cancelled, the synchronizing signal is also eliminated by the arrival of a wide noise including the synchronizing signal or by the channel switching from a broadcasting station of low field intensity to a broadcasting station of high field intensity. As a result, the AGC voltage lowers, and a lockout phenomenon which increases the gains of the video intermediate frequency amplifier stage and the radio frequency amplifier stage takes place.
To the end of preventing this lockout phenomenon from occurring, it has been proposed to employ as the noise detection level a voltage which is formed by subjecting a composite video signal to a level detection.
In this case, however, the detection level responds to the wide noise, and hence, there is the disadvantage that the latter half part of a noise pulse cannot be eliminated. Here, when a time constant for the level detection is made sufficiently great, the same lockout as in the noise cancelling based on the fixed level, as above described, occurs temporarily. Therefore, the time constant cannot be made very great.
In the noise detection based on the level detection voltage, accordingly, although the lockout phenomenon can be prevented, the operation of a synchronizing circuit is disturbed by the inoperative state of the noise canceller as described above and the AGC voltage is adversely affected by the noise.
Further, in the case where the noise canceller is to be contained in a monolithic semiconductor integrated circuit, it increases the number of external terminals and external components required to form with the external components a time constant circuit to be used for the level detection. On the other hand, when the time constant circuit is contained in the monolithic semiconductor integrated circuit, there occurs the problem that the density of integration lowers sharply on account of the presence of a capacitor of large capacitance in the circuit.